Super-cooled liquid water (SLW) droplets form in clouds when water vapor accumulates to saturation levels and condenses on cloud condensation nuclei at temperatures from 0.degree. centigrade to minus forty degrees centigrade. SLW droplets grow by further condensation and by collisions with other drops and coalesce to sizes in excess of 100 micrometers, at which point the droplets begin to fall as precipitation. Over time, therefore, the spectrum of droplet size increases until there are many droplets of different sizes. SLW droplets eventually change to a vapor phase (evaporate) or to a solid (ice) phase (freeze), or they grow to a size sufficient to precipitate. The SLW droplets are a temporary phase, lasting anywhere from less than one minute to several hours.
SLW droplets are formed in specific volumetric zones or regions within clouds. The region of SLW droplets may contain substantially only SLW droplets, or as is the more typical case, it may also contain ice crystals distributed throughout and mixed with the SLW droplets. In such circumstances, the region of SLW droplets and ice crystals is referred to as a mixed phase region.
Ice crystals are less readily and less abundantly nucleated in the atmosphere than are liquid droplets. Therefore, at all but the coldest temperatures, SLW droplets will form before ice crystals. Ice crystals nucleate by several mechanisms, and the rate at which crystals nucleate is a complex function of the ice nuclei concentration, the temperature, the water droplet size distribution, and humidity.
In mixed phase regions the presence of ice crystals tend to deplete the concentration of SLW droplets. Sometimes, an approximate balance is achieved between the rate of creation of SLW droplets by condensation within updrafts and the rate of depletion of SLW droplets by the growing ice crystals. In such circumstances the amount of SLW in the mixed phase region remains nearly constant. In other situations, the condensation rate will not keep up with the depletion rate of SLW droplets caused by the proximity of ice crystals, and the region tends to change from a mixed phase region to one containing only ice crystals. Further still, if the depletion of SLW droplets by the presence of ice particles is not too great, the amount of SLW in the region can increase by further condensation from a cloud updraft, for instance, causing the mixed phase region to change to a region which is predominantly or only SLW droplets.
Sufficient quantities of SLW droplets in the flight path of an aircraft have the potential to adversely impact its flight performance. SLW droplets turn to ice on contact with all components of the aircraft exposed to the airstream and which are not in some way protected. A coating of ice on the leading edges of the wings reduces the amount of lift, increases the amount of drag, and, if the coating is substantial enough, may even cause the aircraft to crash. Statistics suggest the seriousness of aircraft icing incidents. An average of over 60 civilian lives appear to be lost every year due to icing related aircraft accidents. During a five and a half year period between 1978 and 1983, 280 icing-related aircraft accidents occurred with 364 fatalities and 161 injuries.
Current weather warning radars detect precipitation sized particles in clouds, primarily as the result of analyzing return or echo signals which have been reflected by particles of sufficient size to cause a measurable reflection. Generally these weather radars are not sensitive enough to detect liquid droplets that are often only a few tens of micrometers in diameter. Because the reflected signals occur as a result of the particle size, such weather warning systems are not capable of distinguishing between water droplets and ice particles. Even if current weather radar systems had sufficient sensitivity to distinguish between liquid droplets and ice crystals, the signals reflected from the liquid droplets would be masked by the presence of ice crystals in mixed phase clouds. Furthermore, if these weather warning systems are ground based, it is difficult or impossible to determine the temperature of the liquid water droplets. Current airborne weather warning radar systems are subject to the same shortcomings as ground based weather warning systems. In short, current weather warning radar systems cannot distinguish between clouds with hazardous amounts of SLW, or determine whether regions of liquid water are supercooled.
Weather and atmospheric reports are available for pilots to use in navigation, but frequently these weather reports are dated or inaccurate. The rapidity with which the state and size of regions of SLW droplets may change as described above, the inability of weather warning systems to detect regions of SLW droplets, the localized nature of the regions of SLW droplets, and the practical limitation of being unable to communicate changes in atmospheric conditions to pilots as quickly as the those conditions may change, lead to the practical realization that current weather information available to aircraft pilots is of little value in avoiding hazardous icing conditions. At the present time, no warning system is available to pilots to indicate those regions of SLW within clouds in time to divert the course of the aircraft, and to detect conditions which could create a risk of aircraft icing due to the existence of regions of SLW while distinguishing other conditions which pose no threat to aircraft.
It is with respect to this briefly summarized background information and other information that the present invention has evolved.